With stiff sentences for 21 conspirators last week in the United States and a round of indictments in India, the Justice Department says it has broken up what appeared to be the nation’s first large-scale, multinational telephone fraud operation.

Over four years, more than 15,000 victims in the United States lost “hundreds of millions” of dollars to the sophisticated scam, and more than 50,000 individuals had their personal information misused, the department said Friday. The money was routed through call centers in India back to the ringleaders in eight states.

The fraudulent calls came suddenly and frequently while the scam was active from 2012 to 2016, according to court documents. A person posing as an Internal Revenue Service or immigration official was on the phone, threatening arrest, deportation or other penalties if the victims did not immediately pay their debts with prepaid cards or wire transfers.

The calls targeted the most vulnerable Americans, including immigrants and older people.

An 85-year old woman in San Diego paid $12,300 to people claiming to be I.R.S. employees who threatened her with arrest for tax violations.

A Chicago man paid $5,070 after being threatened with arrest and deportation by supposed state police and immigration authorities, the indictment said.

The words “U.S. Government” showed up as the caller I.D. on a number from which a New Hampshire woman was told to pay the I.R.S. $3,980 in payment cards, the court papers said.

In the announcement on Friday, the department said 21 people living in eight states — Illinois, Arizona, Florida, California, Alabama, Indiana, New Jersey and Texas — were sentenced last week in Houston to prison for up to 20 years for their role in the scheme.

Two other conspirators in Illinois were sentenced in February to between two years to just over four years for conspiracy, and a third person in Arizona was given probation in a plea agreement, it said.

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